RF power detectors are commonly used to measure output power in RF communications systems, which need to control output power of RF power amplifiers. The output power is controlled to stay within regulatory power limits, to minimize power consumption in battery powered devices, such as mobile terminals, mobile phones, and Personal Digital Assistants (PDAs), and to transmit only enough power for effective communications, thus minimizing interference with other RF devices.
Gain, DC current, and adjacent channel power (ACP) of an RF power amplifier are functions of the load impedance seen at the output of the power amplifier. Ideally, the load impedance is a constant load; however, in reality, the load impedance may change due to variations in the impedance of the components in the transmit chain following the power amplifier, such as a duplexer, caused by frequency, temperature, and process variations. Load impedance may also vary due to variations in impedance of an antenna of a mobile device caused by proximity of the antenna to foreign metallic objects and a user's body. As a result, RF power amplifier architectures that tend to minimize gain, DC current, and ACP variations due to changes in load impedance are commonly used. Once such architecture is a quadrature RF power amplifier.
The stability of a quadrature RF power amplifier's performance is not as susceptible to output load variations as other RF power amplifier architectures. A quadrature RF power amplifier splits and phase shifts an RF input signal into two quadrature RF signals that are 90 degrees out of phase with each other. The two quadrature RF signals are then amplified, phase shifted back into phase with each other, and then combined to form an RF output signal. The quadrature power amplifier minimizes performance variations by presenting each side of the amplifier with a load that is 180 degrees out of phase with the other side of the amplifier. When one side of the amplifier is presented with a load that causes low gain, the other side is being presented with a load that causes high gain on that side. This is in contrast to a single ended amplifier in which the amplifier stage is presented with a single load. Hence, the quadrature architecture provides a method of diversifying the load presented to the power amplifier. This diversification results in higher immunity to performance variation resulting from a high voltage standing wave ratio (VSWR) at the power amplifier's output.
Traditional approaches for detecting output power use a directional coupler at the output of the RF power amplifier to provide a signal representative of the output of the power amplifier. A power detector then processes the signal to detect the output power of the power amplifier; however, the directional coupler uses some of the output power, which adds loss to the transmit path, thereby decreasing the efficiency of the RF communications system. Thus, there remains a need for a system for detecting the output power of an RF power amplifier that is tolerant to variations in load impedance and eliminates the need for a directional coupler.